Argon is a highly inert element over a very wide range of conditions, both at cyrogenic and very high temperatures. It is used in the steel-making, light bulbs and electronics industries, for welding and in gas chromatography. The major source of argon is that found in the air and it is typically produced therefrom using cryogenic air separation units. The world demand for argon is increasing and thus it is essential to develop an efficient process which can produce argon at high recoveries using cryogenic air separation units.
Historically, the typical cryogenic air separation unit used a double distillation column of the Linde-type with a crude argon (or argon side arm) column to recover argon from air. A good example of this typical unit is disclosed in an article by Latimer, R.E., entitled "Distillation of Air", in Chemical Engineering Progress, 63 (2), 35-39 [1967]). A conventional unit of this type is shown in FIG. 1, which is discussed later in this disclosure.
However, this conventional process has some shortcomings. U.S. Pat. No. 4,670,031 discusses in detail these shortcomings and explains the problems which limit the amount of crude argon recovery with the above configuration. This can be easily explained. For a given production of oxygen and nitrogen products, the total boilup and hence the vapor flow in the bottom-most section (between the bottom of the column and the withdraw line for the crude argon column) of the low pressure column is nearly fixed. As this vapor travels up the low pressure column it is split between the feed to the crude argon column and the vapor proceeding up the low pressure column. The gaseous feed to the top of the section of the low pressure column above the withdraw for the crude argon column (Section 11) is derived by the near total vaporization of a portion of the crude liquid oxygen stream in the boiler/condenser located at the top of the crude argon column. The composition of this gaseous feed stream is typically 35-40% oxygen. A minimum amount of vapor is needed in Section II of the low pressure column--the amount necessary for it to reach the composition at the feed introduction point without pinching in this section. Since the composition of gaseous feed stream is essentially fixed, the maximum flow of vapor which can be sent to the crude argon column is also limited. This limits the argon which can be recovered from this process.
In order to increase argon recovery, it is desirable to increase the flow of vapor to the crude argon column. This implies that the vapor flow through Section II of the low pressure column must be decreased (as total vapor flow from the bottom of the low pressure column is nearly fixed). One way to accomplish this would be to increase the oxygen content of the gaseous feed stream to the top of the Section II of the low pressure column because that would decrease the vapor flow requirement through this section of the low pressure column. However, since this gaseous feed stream is derived from the crude liquid oxygen, its composition is fixed within a narrow range as described above. Therefore, the suggested solution is not possible with the current designs and the argon recovery is thus limited.
U.S. Pat No. 4,670,031 suggests a method to increase the argon recovery and partially overcomes the above discussed deficiency. This is achieved by the use of an additional boiler/condenser. This additional boiler/condenser allows the exchange of latent heats between an intermediate point of the crude argon column and a location in Section II of the low pressure column. Thus a vapor stream is withdrawn from an intermediate height of the crude argon column and is condensed in this additional boiler/condenser and sent back as intermediate reflux to the crude argon column. The liquid to be vaporized in this boiler/condenser is withdrawn from the Section 11 of the low pressure column and the heated fluid is sent back to the same location in the low pressure column. A boiler/condenser is also used at the top of the crude argon column to provide the reflux needed for the top section of this column. A portion of the crude liquid oxygen is vaporized in this top boiler/condenser analogous to the conventional process. The use of the additional boiler/condenser provides some of the vapor at a location in Section II where oxygen content in the vapor stream is higher than that in the crude liquid oxygen stream. This decreases the minimum vapor flow requirement of this section and thereby allows an increased vapor flow to the bottom of the crude argon column. This leads to an increase in argon recovery.
Even though the method suggested in the U.S. Pat. No. 4,670,031 leads to an increase in argon recovery, it is not totally effective. This is due to the fact that all the vapor feed to the crude argon column does not reach the top of this column and an increased L/V is used in the bottom section of this column. Since argon is withdrawn from the top of the crude argon column and a certain L/V is needed in the top section to achieve the desired crude argon purity, the relatively lower vapor flow in the top section (as compared to the bottom section) limits the argon recovery. It is desirable to have a scheme, which will produce an increased vapor flow in the top section of the crude argon column so that argon can be recovered in even greater quantities.
U.S. Pat. No. 4,822,395 teaches another method of argon recovery. In this method all the crude liquid oxygen from the bottom of the high pressure column is fed to the low pressure column. The liquid from the bottom of the low pressure column is let down in pressure and boiled in the boiler/condenser located at the top of the crude argon column. The crude argon column overhead vapor is condensed in this boiler/condenser and provides reflux to this column. There are some disadvantages of this method. The liquid from the bottom of the low pressure column is nearly pure oxygen and since it condenses the crude argon overhead vapor, its pressure when boiled will be much lower than the low pressure column pressure. As a result, the oxygen gas recovered will be at a pressure which is significantly lower than that of the low pressure column and when oxygen is a desired product this represents a loss of energy. Furthermore, this arrangement requires that the low pressure column operates at a pressure which is significantly higher than the ambient pressure. If nitrogen is not a desired product or if it is not needed at a higher pressure then this process will require excessive energy consumption. Another drawback of the suggested solution is that since crude argon overhead is condensed against pure oxygen, the amount of vapor which can be fed to the crude argon column is limited by the amount of oxygen present in the air. In some cases, this can lead to lower argon recoveries.
There is clearly a need for a process which does not have above mentioned shortcomings and can produce argon with greater recoveries.